Cylindrical filter formed of stacked fibers having a high and low melting point component

ABSTRACT

A cylindrical filter formed of stacked microfine, conjugate fibers consisting of a higher melting point component and a lower melting point component, the fiber diameter of the conjugate fibers being varied into a smaller one in the thickness direction of the filter and along with the passing direction of a fluid to be filtered, and the contact points of the conjugate fibers being melt-adhered by the lower melting point component is provided. This cylindrical filter is produced by a process of subjecting a higher melting point component and a lower melting point component each consisting of a fiber-forming thermoplastic polymer to conjugate melt-blow spinning so that the fiber diameter is varied during the spinning into a smaller one in the thickness direction of the filter and along with the passing direction of the fluid, winding up and stacking the fibers on a core to form a cylindrical shape on the core, heat-treating the resulting web at a temperature higher than the melting point of the lower melting point component and lower than the melting point of the higher melting point component, before and/or at the time of or after the winding-up, and drawing out the core.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a cylindrical filter for precision filtration,prepared by winding up microfine fibers according to a melt-blow processin a cylindrical form, and a process for producing the same.

2. Description of the Related Art

Various kinds of filters have been known, which are obtained by moldingsynthetic fibers into a cylindrical form. Japanese patent publicationNo. Sho 56-43139 discloses a process of winding up a carded web ofconjugate fibers onto a core under heating. However, according to such aprocess, it is difficult to subject fine fibers of 1d/f or less to acarding process stably. Hence, a filter collecting fine particles of 10μm or less could not have been obtained. Further, in the case ofconventional synthetic fibers, an oiling agent is coated on the fibersin order to prevent charge and abrasion during the steps of spinning,stretching, carding, etc. This oiling agent causes such problems that itelutes from the fibers into the filtrated during filtration, resultingin bubbling and contaminating the filtrate such as foods, etc.

On the other hand, as cylindrical filters for precision filtration,filters using microfine fibers according to a melt-blow process havebeen broadly used as a filter for cleaning solutions for materials ofelectronic equipments or as an air filter for dedusting or as aprefilter for water, etc. used for pharmaceutical products, etc.

Japanese patent application laid-open No. Sho 60-216818 discloses aprocess of winding up fibers obtained according to a melt-blow processon a core after the fibers have been cooled down to a temperature atwhich they are not adhered to one another, and also discloses a processof gradually varying the fiber diameter in the thickness direction ofthe filter by controlling the spinning conditions. In the case of such afilter, the fibers are adhered to one another by entanglements of thefibers almost without bonding of the fibers with one another. Hence, thehardness of the filter is so low that a sufficient pressure-resistancecannot be obtained. In order to increase the hardness, a process ofwinding up the web while heating it may be considered, but the web ischanged into a film due to melting of the fibers, so that clogging ofthe filtering layer occurs or the size of voids in the filter becomesnon-uniform, resulting in a product having an inferior filtration lifeand accuracy.

Japanese patent application laid-open No. Hei 1-297113 discloses aprocess of winding up several kinds of non-woven fabrics according to amelt-blow process, each having different fiber diameters and bulkdensities, successively each several times so that the inner layer ofthe filter may be dense and the outer layer thereof may be rough.However, according to the process, it is necessary to prepare severalkinds of non-woven fabrics in advance, and not only the production stepsare complicated and not efficient, but also the resulting filter is notadhered between the respective fibers of the non-woven fabric andbetween the respective layers thereof. Hence, solution leakage from theend part of the filter due to peeling-off of the layers during use ofthe filter are liable to occur and the pressure resistance isinsufficient.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a filter for precisionfiltration, which has a superior pressure resistance, does notcontaminate the filtrate and has a long filtration life, and a simpleprocess for producing the same.

The present inventors have made extensive research in order to solve theabove-mentioned problems, and as a result, have found that the objectcan be achieved by winding up a microfine, conjugate fibers web obtainedby stacking the fiber. According to a melt-blow process, whilesuccessively varying the fiber diameter, onto a core, heating the web soas to melt only the lower melting point component, to achieve thepresent invention.

The present invention has the following two aspects:

(1) A cylindrical filter formed of stacked microfine, conjugate fibersconsisting of a higher melting point component and a lower melting pointcomponent, each obtained by a melt-blow spinning process, the fiberdiameter of the conjugate fibers being varied into a smaller one in thethickness direction of the filter and along with the passing directionof a fluid to be filtered, and the contact points of the conjugatefibers being melt-adhered by the lower melting point component.

(2) A cylindrical filter according to item (1), wherein the fiberdiameter of the conjugate fibers is varied from 0.5-3.0 micron on theinner surface of the filter, to 2.5-10 micron on the outer surface ofthe filter.

(3) A process for producing a cylindrical filter, which comprisessubjecting a higher melting point component and a lower melting pointcomponent each consisting of a fiber-forming thermoplastic polymer toconjugate melt-blow spinning so that the fiber diameter is varied duringthe spinning into a smaller one in the thickness direction of the filterand along with the passing direction of a fluid to be filtered, windingup and stacking the fibers on a core to form a cylindrical shape on thecore, heat-treating the resulting web at a temperature higher than themelting point of the lower melting point component and lower than themelting point of the higher melting point component, before and/or atthe time of or after the winding-up, and drawing out the core.

(4) A process for producing a cylindrical filter according to item (3),wherein a pressurized air is blown at the melt-blow spinning so that thepressure of the air is decreased continuously or stepwisely during thespinning.

(5) A process for producing a cylindrical filter according to item (3),wherein an extrusion quantity of the polymer extruded from spinningnozzles is increased continuously or stepwisely during the spinning.

(6) A process for producing a cylindrical filter, which comprisessubjecting a higher melting point component and a lower melting pointcomponent each consisting of a fiber-forming thermoplastic polymer toconjugate melt-blow spinning, stacking resulting fibers in the form ofwebs, winding up and stacking two or more of said webs on a core to forma cylindrical shape thereon, and drawing out the core; the fiberdiameters of the webs being varied in the thickness direction of thefilter and along with the passing direction of a fluid to be filtered,and the webs being heat-treated at a temperature higher than the meltingpoint of the lower melting point component and lower than the meltingpoint of the higher melting point component, before and/or at the timeof or after said winding-up and stacking of the webs on the core.

(7) A process for producing a cylindrical filter according to item (6),wherein the webs before the winding up and stacking thereof on the coreare formed into non-woven fabrics.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention will be described in detail.

The cylindrical filter referred to herein means a cylindrical filterhaving a cross-section of circular shape, elliptical shape, etc., aswell as of multiple angles shape like triangular shape, tetrangularshape, etc. In addition, in the case where the shape of the core ispolygonal (hexagonal, octagonal, etc. for example), as the fiber web issuccessively wound up and stacked on the core, the polygonal outer shapeof the web is liable to become rounder and be close to a circular shape,but this has no influence upon the filter characteristics.

The conjugate fibers used for the filter of the present invention areobtained by subjecting two kinds of resins having different meltingpoints of preferably 20° C. or more difference selected fromfiber-forming thermoplastic resins such as polyolefins, polyesters,polyamides, etc. to conjugate spinning into side-by-side form orsheath-and-core type wherein the low melting point resin is arranged onthe sheath side. In the case of the sheath-and-core type, not only astructure having a eccentric sheath component, but also a structurehaving a plurality of core components may be employed. Further, it isalso possible to render the fiber cross-section as circular, ellipticalor other various shaped cross-sections. An important factor for thestructure of conjugate fibers consists in that the lower melting pointcomponent occupies at least a portion of the periphery of the fibercross-section. The proportion in which the lower melting point componentoccupies the periphery of the fiber cross-section is not limited, andmay vary in the direction of the fiber axis, for example. In short, sucha conjugate structure may be formed so that the lower melting componentcauses melt-adhesion at the respective contact points of the fibers byheat-treatment as described below. Examples of the higher melting pointcomponent and the lower melting point component subjected to conjugatespinning are as follows:

combinations of polyethylene/polypropylene, polypropylene/polyester,nylon 6/nylon 66, etc. If the difference of the melting points of boththe resins is less than 20° C., the temperature range of the heattreatment becomes narrow so that the process control becomes difficult.In addition, the conjugate ratio by weight of both the components areusually 80/20 to 20/80, preferably 65/35 to 35/65, more preferably 45/55to 55/45.

As a process for spinning such conjugate fibers according to a melt-blowprocess, a process may be employed wherein, using a spinneret device asdisclosed in Japanese patent application laid-open No. Sho 60-99057, twokinds of thermoplastic resins are melt and extruded from the respectiveextruders into a nozzle plate, followed by blowing off the melted resinextruded from spinning nozzles by a high speed hot air and stacking theresulting microfine conjugate fibers on a collecting conveyer.

As to the change in the fiber diameter in a melt-blow process, thediameter becomes larger with increase in the quantity of the resinextruded, while it becomes smaller with increase in the flow rate of thehot air. Hence, when either one or the both of these conditions aresuccessively or stepwisely varied, it is possible to obtain a web havingthe fiber diameter varied successively or stepwisely in the thicknessdirection of the web.

In the present invention, the diameter is successively or stepwiselydecreased in the thickness direction of the filter and along with thepassing direction of a fluid to be filtered. That is, when the passingdirection of the fluid is from the outer surface of the cylindricalfilter toward its inner surface thereof, the fiber diameter is madesuccessively smaller from the outer surface of the filter toward itsinner surface. To the contrary, when the passing direction is from theinner surface of the filter toward its outer surface, the fiber diameteris made successively smaller from the inner surface toward its outersurface. By employing such structure, the resulting filter has largervoids formed by thicker fibers on the inlet side of the solution, whileit has smaller voids formed by finer fibers on the exit side of thesolution (a density-gradient type filter is obtained). Thus, fineparticles are classified and caught in the thickness direction of thefilter, so that it is possible to obtain a filter having a long life offiltration.

As to concrete changes of the fiber diameter in the thickness directionof the filter, the fiber diameter is preferably varied from 2.5-10micron (more preferably 3-8 micron) on the inlet surface of the filter,to 0.5-3.0 micron (more preferably 0.8-2.5 micron) on the outlet surfaceof the filter.

In the present invention, when a microfine conjugate fiber web obtainedby stacking the fibers while varying the fiber diameter in the thicknessdirection is wound up and stack on a core to produce a cylindricalfilter, heat-treatment is carried out before and/or at the time of orafter the winding-up. The heat treatment referred to herein meansheating at a temperature higher than the melting point of the lowermelting point component and lower than the melting point of the highermelting point component. Concretely, the microfine conjugate fiber webhaving the fiber diameter successively varied, obtained by a melt-blowprocess, is once heat-treated into a non-woven fabric form and stored,followed by again heating it before or at the time of or afterwinding-up on a core, to obtain the cylindrical filter of the presentinvention. Alternatively, the web obtained after spinning is heatedbefore and/or at the time of or after winding-up on a core, to obtainthe cylindrical filter.

By such a heat-treatment, the contact points of the conjugate fiberswith one another inside the web are fixed and also the layers of thewound webs are fixed therebetween by melt-adhesion of the lower meltingcomponent of the conjugate fibers. Hence, it is possible to obtain afilter having a high pressure-resistance.

As to the effectiveness of such a heat-treatment, it is more preferableto carry out the heat-treatment before and/or at the time of thewinding-up on a core (at the winding-up point). In this case, since thefibers start to fix with each other from the inner layer of the web, thefilter is sufficiently endurable to the outer pressure loaded at thetime of winding up on a core, and also, the inner layer is liable toform denser voids between the fibers. Thus, the density gradient becomesnotable along with the above-mentioned change in the fiber diameter, sothat a synergistic effect of superior pressure-resistance and fineparticles-classifying and collecting function can be obtained.

In general, filters are compressed by the pressure of a fluid passingtherethrough and the voids between fibers cause clogging thereof toshorten the filtration life. This tendency becomes notable with increasein the fluid viscosity. Whereas, according to the present invention,conjugate fibers consisting of a higher melting point component and alower melting point component are heat-treated, thereby melt-adheringonly the lower melting component at the contact points of the fibers;hence the resulting filter forms a three-dimensional structure by theadhesion at the contact points of the fibers and this structure preventsclogging of the voids brought about by the fluid pressure. Thus, in thepresent invention, porous substrate, reinforcing material, etc. are notrequired for the inner layer part, the filtration accuracy is stabilizeddue to the superior pressure resistance and the filtration life isprolonged. In the filter structure, if such a construction that only thelower melting point component of the conjugate fibers is adhered at thecontact points of the fibers to form a three-dimensional structure, isdeficient, even if the fiber diameter should have been varied in thethickness direction of the filter, the fusion and deformation of thefibers in the filter occur to cause clogging and lead to a shortfiltration life; hence the effectiveness of the present invention cannotbe obtained.

As heating sources for the heat-treatment, hot air, pressurized steam,super-heated steam, far infrared rays heater, etc. are used, but amongthem, far infrared rays heater is particularly preferred, since auniform heat-treatment is carried out without disturbing the web. As toan extent of the heat-treatment, it is adjusted so that fiber-fixing dueto adhesion and a desired voids density may be obtained, that is, thetemperature of a heating zone, the length of a heating zone or a passingspeed i.e. a retention time in the heating zone, etc. are controlled.The web wound up on a core, and having been finished the heat-treatmentis cooled by allowing it to stand at a room temperature, followed bydrawing out the core and cutting to a suitable length to obtain acylindrical filter.

Further, when the web is prepared according to a melt-blow process, fineparticles such as those of active carbon, zeolite, ion exchange resin,etc. or functional fibers such as carbon fibers, sterilizing fibers,gas-adsorbing fibers, etc. may be mixed in the web or between the webswhen they are wound up, within a range wherein the effectiveness of thepresent invention is not prevented. Thus, it is also possible to preparea filter provided with other functions than collecting of fineparticles.

The present invention will be concretely described by Examples andComparative examples. The measurement methods employed in the respectiveexamples will be described below.

Filtration Accuracy

A circulating type, filtration testing apparatus consisting of a watertank containing 30 l of water, a pump and a filtering device isemployed. One sample filter is fixed to the housing of the filteringdevice. While water is circulated in a flow quantity of 30 l/min., 5 gof a cake (carborundum #4000) is added to the water tank. Filtered water(100 ml) collected one minute after addition of the cake is furtherfiltered by a membrane filter capable of collecting particles of 0.6micron or larger. The size of particles collected on the membrane filteris measured by an instrument for measuring the particle sizedistribution, and the size of the largest particle (the largest diameter(micron) of flown-out particles) is regarded as the filtration accuracyof the sample filter.

Pressure Resistance and Filtration Life

One sample filter is fixed to a circulating type, filtration-testingapparatus and water is circulated in a flow quantity of 30 l per min. Ina water tank is added powder of subsoil of volcanic ash soil (averageparticle diameter: 12.9 microns; powder having particle diameters withina range of 1.0 to 30 microns: 99% by weight or more) (20 g), followed bycontinuing circulating filtration. When water in the water tank hasbecome transparent, the difference between the pressures before andafter the filtration is measured. The operations of the addition of thepowder and the measurement of the pressure difference are repeated tillthe filter is deformed (or the pressure difference reaches 10 Kg/cm²).The time since the first powder addition till the filter deformation isregarded as a filtration life, and the pressure difference at that timeis regarded as a pressure resistance.

Average Fiber Diameter

Ten thin pieces (each 4 cm×4 cm) are respectively cut from a web, anon-woven fabric or a filter, and the diameters of 100 ends of therespective fibers are measured with photographs of a magnification of5,000 times taken by a scanning type electronic microscope. The averageof the thus measured diameters is regarded as average fiber diameter.

EXAMPLE 1

Using a spinneret device for sheath-and-core type, conjugate melt-blowspinning having spinning nozzles (each, nozzle diameter: 0.3 mm andnumber of nozzles: 501) arranged in a row, polypropylene (melt flow rate(MFR: 230° C.): 280 g/10 min. and m.p.: 164° C.) as a core component(spinning temperature: 290° C.) and a linear, low density polyethylene(melt flow rate (MFR: 190° C.): 124 g/10 min. and m.p.: 122° C.) as asheath component (spinning temperature: 260° C.) were extruded in asheath-and-core conjugate ratio of 50/50 and in a total extrusionquantity of 120 g/min., blowing extrudates from the spinning nozzlesonto a net conveyer by means of a pressurized air at 380° C., to obtaina microfine conjugate fiber web according to a melt-blow process. Thepressurized air fed to the spinning nozzles was continuously andgradually reduced from an initial pressure of 3.2 Kg/cm² G down to afinal period pressure of 0.6 Kg/cm² G.

This web was heated up to 145° C. by means of a far infrared rays heaterwhile transferring it by a net conveyer, followed by winding up the webon a circular stainless pipe (outer diameter: 30 mm) and allowing theresulting web to cool at a room temperature, as disclosed in Japanesepatent publication No. Sho 56-43139. Thereafter, the stainless pipe wasdrawn out, followed by cutting the web to a length of 250 mm, to obtaina cylindrical filter (inner diameter: 30 mm, outer diameter: 60 mm andlength: 25 mm).

The measurements of the samples taken from the web were as follows:

the average fiber diameters at the respective parts in the thicknessdirection of the filter: 0.8 micron (μm) on the inner surface, 1.8micron (μm) at 5 mm apart from the inner surface, 2.7 micron (μm) at 10mm apart from the inner surface, and 7.6 microns, on the outer surface.

In this filter, the respective fibers were adhered to one another at thecontact points by melt-adhesion of the polyethylene as the lower meltingpoint component to form a three-dimensional hard structure, and evenwhen the filter was struck onto a desk, it was not deformed. This filterexhibited a filtration accuracy of 0.9 micron, a pressure resistance of6.3 Kg/cm² and a filtration life of 30 min., and foaming of the filtrateat the initial period of the filtration was not observed.

EXAMPLE 2

Using the same spinneret device as in Example 1, polyethyleneterephthalate (intrinsic viscosity: 0.60, m.p.: 253° C., and spinningtemperature: 285° C.) as a core component, and ethylene glycolterephthalate.isophthalate copolymer (intrinsic viscosity: 0.58, m.p.:160° C., and spinning temperature: 270° C. as a sheath component, wereextruded in a sheath-and-core conjugate ratio of 50/50 and in a totalextrusion quantity of 120 g/min., blowing the polymers extruded from thespinning nozzles onto a net conveyer by a pressurized air at 350° C., toobtain a microfine conjugate fiber web according to a melt-blow process.The pressurized air fed to the spinning nozzles was continuously andgradually decreased from the initial pressure of 2.8 Kg/cm² G down tothe final period pressure of 0.4 Kg/cm² G. This web was heated up to170° C. by a far infrared rays heater, while transferring it by the netconveyer, followed by winding up it on a circular stainless pipe of anouter diameter of 30 mm and allowing it to cool at a room temperature.Thereafter, the stainless pipe was drawn out, followed by cutting it toa length of 250 mm, to obtain a cylindrical filter (inner diameter: 30mm, outer diameter: 60 mm and length: 250 mm).

The average fiber diameters at the respective parts in the thicknessdirection of the filter were 1.8 micron on the inner surface, 3.9microns at 5 mm apart from the inner surface, 6.8 microns at 10 mm apartfrom the inner surface and 9.2 microns on the outer surface. The fiberswere adhered to one another at the contact points thereof bymelt-adhesion of the lower melting point component to form athree-dimensional structure. The filter exhibited a filtration accuracyof 1.6 micron, a pressure resistance of 7.4 Kg/cm² and a filtration lifeof 36 min. Foaming of filtrate at the filtration initial period was notobserved at all.

EXAMPLE 3

Using a spinneret device for sheath-and-core type conjugate, melt-blowspinning having spinning nozzles (nozzle diameter: 0.3 mm and 501nozzles) arranged in a row, polypropylene (melt flow rate (MFR: 230°C.): 204 g/10 min., m.p.: 165° C. and spinning temperature: 280° C.) asa core component, and a linear, low density polyethylene (melt flow rate(MFR: 190° C.): 124 g/10 min., m.p.: 122° C. and a spinning temperature:240° C.) as a sheath component were extruded in a core-and-sheathconjugate ratio of 50/50 and in a total extrusion quantity of initially120 g/min. and from midway, 160 g/min. The polymers extruded fromspinning nozzles were blown onto a net conveyer by a pressurized air of1.9 Kg/cm² G, to obtain a microfine conjugate fiber web according to amelt-blow process.

This web was successively heated up to 145° C. by a far infrared raysheater while transferring it by the net conveyer, followed by winding upthe web onto a circular stainless pipe (outer diameter: 30 mm) andallowing it to cool at a room temperature, as disclosed in Japanesepatent publication No. Sho 56-43139. Thereafter the stainless pipe wasdrawn out, followed by cutting the web to a length of 250 mm, to obtaina cylindrical filter (inner diameter: 30 mm, outer diameter: 60 mm andlength: 250 mm).

The average diameters at the respective parts of the filter were 1.8micron from the inside surface up to 9 mm, and 2.7 microns at 9 mm ormore apart from the inside surface. The fibers were adhered to one otherat their contact points by adhesion of the low melting component to forma three-dimensional structure. This filter exhibited a filtrationaccuracy of 2.6 microns, a pressure resistance of 6.1 Kg /cm² andfiltration life of 30 minutes. Foaming of the filtrate at the initialperiod of filtration was not observed.

EXAMPLE 4

Using a spinneret device for sheath-and-core type, conjugate, melt-blowspinning, having spinning nozzles (nozzle diameter: 0.3 mm and 501nozzles) arranged in a row, polypropylene (melt flow rate (MFR: 230°C.): 204 g/10 min., m.p.: 165° C. and spinning temperature: 280° C.) asa core component and a linear, low density polyethylene (melt flow rate(MFR: 190° C.): 124 g/10 min., m.p.: 122° C. and spinning temperature:240° C.) as a sheath component, were extruded in a core-and-sheathconjugate ratio of 70/30 and in a total extrusion quantity of 120g/min., blowing the polymers extruded from the spinning nozzles onto anet conveyer by a pressurized air of 1.9 Kg/cm² G and 360° C., to obtaina microfine conjugate fiber web (average fiber diameter: 2.4 microns)according to a melt-blown process. This web was wound up through aheating vessel heated to 140° C. by a far infrared rays heater, toobtain a non-woven fabric (non-woven fabric A) (average fiber diameter:2.4 microns).

In the same manner as the above, except that the total extrusionquantity was changed to 160 g/min., a non-woven fabric (non-woven fabricB) (average fiber diameter: 8.8 mm) was obtained.

The non-woven fabric A was wound up on a stainless pipe (outer diameter:30 mm) up to a thickness of 10 mm, while heating it at 145° C. by a farinfrared rays heater, followed by winding up the non-woven fabric B upto a thickness of 5 mm while heating it in the same manner as the above,and allowing it to cool at a room temperature. Thereafter, the stainlesspipe was drawn out to obtain a cylindrical filter (inner diameter: 30mm, outer diameter: 60 mm and length: 250 mm).

This filter, too, had fibers adhered to one another at the contactpoints thereof by melt-adhesion of the lower melting component to afforda hard product of a three-dimensional structure (filtration accuracy:2.4 microns, pressure resistance: 7.7 Kg/cm² and filtration life: 30min.). Foaming of the filtrate at the initial period of filtration wasnot observed.

EXAMPLE 5

Using a spinneret device for side by side type, conjugate, melt-blowspinning, having spinning nozzles (nozzle diameter: 0.3 mm and 501nozzles) arranged in one row, polypropylene (melt flow rate (MFR: 230°C.): 280 g/10 min., m.p.: 164° C. and spinning temperature: 290° C.) asa first component and a linear, low density polyethylene (melt flow rate(MFR: 190° C.): 124 g/10 min., m.p.: 122° C. and spinning temperature:260° C.) as a second component, were extruded in a conjugate ratio of60/40 and in a total quantity extruded, of 120 g/min., blowing thepolymers extruded from the spinning nozzles onto a net conveyer by apressurized air at 380° C., to obtain a microfine, conjugate fiber webaccording to a melt-blow process. The pressurized air fed to thespinning nozzles was continuously and gradually decreased from theinitial 3.2 Kg/cm² G down to the final 0.6 Kg/cm² G.

This web was heated to 145° C. by a far infrared rays heater, whiletransferring it by a net conveyer, followed by winding up it onto acircular stainless pipe (outer diameter: 30 mm) and allowing it to coolat room temperature, as disclosed in Japanese patent publication No. Sho56-43139. Thereafter, the stainless pipe was drawn out, followed bycutting to a length of 250 mm, to obtain a cylindrical filter (innerdiameter: 30 mm, outer diameter: 60 mm and length: 250 mm).

Measurements of the average fiber diameters at the respective parts inthe thickness direction of samples taken from the web were as follows:

0.9 micron on the inner surface, 1.6 micron at 5 mm apart from the innersurface, 2.8 microns at 10 mm apart from the inner surface and 7.3microns on the outer surface. In this filter, the fibers were adhered toone another at the contact points thereof by melt-adhesion ofpolyethylene as the lower melting point component to form athree-dimensional structure, and even when it was struck onto a desk, itwas a hard product without deformation. This filter exhibited afiltration accuracy of 0.9 micron, a pressure resistance of 6.1 Kg/cm²and a filtration life of 29 min. Foaming of the filtrate at the initialperiod of filtration was not observed.

EXAMPLE 6

A microfine, conjugate fiber web according to a melt-blow process,obtained by spinning in the same conditions as in Example 5, was heatedto 145° C. by a far infrared rays heater, while transferring it by a netconveyer, followed by winding up it on a stainless pipe having aperiphery of a right hexagonal shape (width of each side: 15 mm), andallowing it to cool at a room temperature. Thereafter, the stainlesspipe was drawn out, followed by cutting the web to a length of 250 mm,to obtain a cylindrical filter. This filter had an outer diameter of 60mm at the largest part and that of 52 mm at the least part, that is, ithad a shape almost close to a circular shape.

In this filter, the fibers were adhered to one another at the contactpoints thereof by adhesion of polyethylene to form a three-dimensionalstructure, and even when the filter was struck onto a desk, it was ahard product without deformation. This filter exhibited a filtrationaccuracy of 0.9 micron, a pressure resistance of 5.7 Kg/cm² and afiltration life of 30 min. Foaming of the filtrate at the initial periodof filtration was not observed.

EXAMPLE 7

Using a spinneret device for sheath-and-core type, conjugate melt-blowspinning, having spinning nozzles (nozzle diameter: 0.3 mm and 501nozzles) arranged in a row, polypropylene (melt flow rate (MFR: 230°C.): 180 g/10 min., m.p.: 165° C. and spinning temperature: 280° C.) asa core component and propylene-ethylene-butene-1 random copolymer (meltflow rate (MFR: 190° C.): 135 g/10 min., m.p.: 138° C. and spinningtemperature: 300° C.) as a sheath component were extruded in acore-sheath conjugate ratio of 50/50 and initially in a total extrusionquantity of 120 g/min. and increasing from midway, in a total extrusionquantity of 160 g/min.

A core made of a porous pipe was rotated at a peripheral speed of 10m/min. while the inside of the pipe was evacuated by suction, and thepolymers extruded from the spinning nozzles were blown onto the core bya pressurized air (temperature: 360° C. and pressure: 1.9 Kg/cm²),whereby a microfine conjugate fiber web according to a melt-blow processwas stacked around the core and wound up thereon.

After completion of the winding up, while the suction and rotation werecontinued, the web together with the core were heated in a heatingvessel having the atmospheric temperature of 140° C. by a far infraredrays heater, followed by allowing the web to cool at a room temperature.Thereafter, the core was drawn out, followed by cutting the web to alength of 250 mm, to obtain a cylindrical filter (inner diameter: 30 mm,outer diameter: 60 mm and length: 250 mm).

Measurements of samples taken from the web were as follows:

the average fiber diameters at the respective parts in the thicknessdirection of the filter were 1.6 micron between the inside surface and 9mm apart therefrom, and 2.8 microns at 9 mm or more apart from theinside surface. The fibers were adhered to one another at their contactpoints by melt-adhesion of the lower melting point component to form athree-dimensional structure. Even when the filter was struck onto adesk, it was a hard product having no deformation. This filter exhibiteda filtration accuracy of 2.5 microns, a pressure resistance of 6.8Kg/cm² and a filtration life of 27 min. Foaming of the filtrate at theinitial period of filtration was not observed.

Comparative Example 1

Using a spinneret device for melt-blow spinning, having spinning nozzles(nozzle diameter: 0.3 mm and 501 nozzles) arranged in a row,polypropylene (melt flow rate (MFR: 230° C.): 180 g/10 min. and m.p.:164° C.) was extruded at a spinning temperature of 280° C. and in anextrusion quantity of 120 g/min., blowing the polymer extruded from thenozzles onto a net conveyer by a pressurized air of 380° C., to obtain amicrofine fiber web according to a melt-blow process. The pressurizedair fed to the spinneret was continuously and gradually decreased fromthe initial 3.2 Kg/cm² G down to the final 0.6 Kg/cm² G.

In the same manner as in Example 1, this web was heated at 190° C. by afar infrared rays heater, while transferring it by a net conveyer,followed by winding up it on a circular stainless pipe (outer diameter:30 mm), allowing it to cool at a room temperature, drawing out thestainless pipe and cutting the web to a length of 250 mm, to obtain acylindrical filter (inner diameter: 30 mm, outer diameter: 60 mm andlength: 250 mm).

The average fiber diameters at the respective parts of the web in thethickness direction were 3.4 microns on the inner surface, 8.2 micronsat 5 mm apart from the inner surface, 15 microns at 10 mm apart from theinner surface and 22 microns on the outer surface, but in the filteritself, cloggings due to fusion and deformation of the fibers wereobserved, so that the filter was very hard. In this filter, foaming ofthe filtrate at the initial period of the filtration was not observed.The filter exhibited a filtration accuracy of 3.2 microns and a pressureresistance of 6.5 Kg/cm², but the filtration life was as very short as 5min.

Comparative Example 2

A cylindrical filter was prepared under the same conditions as inExample 1 except that the pressure of the pressurized air was fixed to3.2 Kg/cm² G. The filter was composed of conjugate fibers having anaverage fiber diameter of 0.9 micron, uniformly in the thicknessdirection of the filter, and had an inner diameter of 30 mm, an outerdiameter of 60 mm and a length of 250 mm. This filter exhibited afiltration accuracy of 0.9 micron and a pressure resistance of 6.5Kg/cm² but its filtration life was as short as 10 min.

Comparative Example 3

A cylindrical filter was prepared in the same conditions as in Example 1except that the pressure of the pressurized air was fixed to 0.6 Kg/cm²G. The filter was composed of conjugate fibers having an average fiberdiameter of 7.3 micron, uniformly in the thickness direction of thefilter, and had an inner diameter of 30 mm, an outer diameter of 60 mmand a length of 250 mm.

This filter exhibited a pressure resistance of 6.05 Kg/cm² and afiltration life of 50 min., but its filtration accuracy was as inferioras 7.0 microns.

Reference Example 1

A polypropylene web of an average fiber diameter of 1.3 micron, obtainedaccording to a melt-blow spinning process, was wound up on a reinforced,porous plastic cylinder to obtain a commercially available filter (innerdiameter: 30 mm, outer diameter: 60 mm and length: 250 mm), and itsproperties were tested.

As to this filter, foaming of the filtrate at the initial period was notobserved, but its average fiber diameter was small and nevertheless itsfiltration accuracy was as inferior as 9.0 microns, its pressureresistance was as low as 1.8 Kg/cm² and its filtration life was as shortas 8 min. It is considered that the reason will be in that since thefibers are fixed to one another merely by a frictional force, the poresinside the filter are opened by the water pressure to lower thefiltration accuracy, or the filter itself is deformed.

Effectiveness of the Present Invention

According to the filter of the present invention, since microfine,conjugate fibers by a melt-blow process and a large gradient of the sizeof the voids between the inside and the outside of the filter by varyingthe fiber diameter in the thickness direction of the filter, thefiltration accuracy is superior and the filtration life is prolonged.Further, since the fibers are adhered to one another at the contactpoints thereof by adhesion of the lower melting point component to forma three-dimensional structure, the filter is hard by itself, noreinforcing material is required and even in the case of a high pressurefiltration, the pores of the filter are not opened, so that the pressureresistance is superior and a precision filtration can be stably carriedout. Further, since an oiling agent for fiber processing is not used,the filtrate is not contaminated by the oiling agent, so that the filteris safely usable in the food-processing field or in the field ofelectronic instruments,

What we claim is:
 1. A cylindrical filter formed of stacked microfine,conjugate fibers consisting of a higher melting point component and alower melting point component, each obtained by a melt-blow spinningprocess, the fiber diameter of the conjugate fibers being varied into asmaller one in the thickness direction of the filter and along with thepassing direction of a fluid to be filtered, and the contact points ofthe conjugate fibers being melt-adhered by the lower melting pointcomponent, said filter having a pressure resistance of at least about5.7 Kg/cm², a filtration life of at least about 30 minutes and afiltration accuracy of at least about 0.9 microns.
 2. The cylindricalfilter of claim 1 wherein the said filter has a pressure resistance ofat least about 6.3 Kg/c².